1. Field of the Invention
The present invention relates to an electrophotographic printer. More particularly, the present invention relates to a method of optimizing the amount of toner supplied from a toner cartridge to a photoconductor of an electrophotographic printer.
2. Description of the Related Art
Generally, electrophotographic printers are operated in the following manner. An exposure unit makes an electrostatic latent image that corresponds to an image signal onto a photoconductor. A toner is applied to the electrostatic latent image to form a toner image. The toner image is transferred and fused onto a printing medium, such as paper, envelopes, labels, and transparencies.
The electrophotographic printer includes a photoconductor, toner cartridges (designated as Y, C, M, K in FIG. 1), and an image transfer unit. An electrostatic latent image is formed on the photoconductor. The toner cartridges are spaced a predetermined distance apart from the photoconductor to supply toner to the electrostatic latent image to develop a toner image. The image transfer unit transfers the toner image from the photoconductor to a printing medium.
The toner cartridge containing the toner is detachably installed in a main body of the printer. Each of the toner cartridges includes an agitator for stirring the toner, a development roller for supplying the toner to the electrostatic latent image of the photoconductor, and a supply roller for supplying the toner to the development roller.
When the toner in the toner cartridge is completely spent, the toner cartridge is replaced with a new one. However, there are several problems with using the replaced toner cartridge.
First, toner density (toner per unit area, M/A [g/cm2]) of the development roller increases in proportion to the number of the printing media that have been printed after replacing the toner cartridge with a new one. The toner density of the development roller increases as the development roller is used for a long time because of stresses exerted on the toner by an agitator roller and the supply roller, selection phenomena of preferentially developing small-sized particles of the toner, or decreases in specific charge of the toner.
The increase of the toner density of the development roller causes increase of scattered toner even though an image density sensor is used to control toner image density of the photoconductor.
Second, the increase of the toner density on the development roller increases the amount of the toner on the photoconductor. The density sensor reads the amount of the toner and a controller optimizes developing conditions to reduce toner density. However, when the toner density increases more than a critical value, the developing condition cannot be optimized by adjusting a developing high voltage. It is difficult to compensate the density by controlling the image density because the replacement of the toner cartridge increases toner density more than the developing condition controlling maintains a constant density.
The increase of scattered toner increases contamination in the printer and the scattered toner may contaminate an optical sensor, thereby reducing the performance of the sensor.
Third, the increase of toner density on the development roller increases toner consumption in proportion to the number of the printing media that have been printed. The increase of the toner consumption lowers the toner transferring efficiency to the printing medium and increases the toner required to print the same number of printing media, thereby increasing costs and decreasing the life span of the toner cartridge.
The above-mentioned problems also increase the size of the development unit, thereby increasing the size of a printer engine. Further, a larger waste toner collector is required or the waste toner collector has to be replaced frequently owing to the increase of the toner consumption.
Accordingly, a need exists for a electrophotographic printer that optimizes toner use to decrease costs and extend the life of toner cartridges.